A -Fluoropyridinium salts

The A -fluoropyridinium salts produce 2-fluoropyridines on treatment with base. A carbene mechanism has been proposed [78] (equation 36). 

Both stereoselectivity and regioselectivity occur in the reaction of steroid vinyl esters, ethers, and related compounds with A -fluoropyridinium salts [75, 7d] (equation 45). 

Thiols have also been prepared from alcohols. One method involves treatment with H2S and a catalyst such as A1203,747 but this is limited to primary alcohols. Another method involves treatment with Lawesson s reagent (see 6-11).748 Still another method, involving the use of a fluoropyridinium salt and sodium N,N-dimethylthiocarbamate, can be applied... 

The main use of A -fluoropyridinium salts is as fluorinating agents. A recent study of A-fluoropyridinium-2-sulfonates substituted with alkyl or trifluoromethyl groups reveals them to be powerful and selective fluorinating... 

The A-fluoropyridinium salts are crystalline species with melting points ranging from 90°C for lc to above 300°C for le (86TL3271). With the notable exception of perchlorates, such as If, they are shock-resistant, thermally stable, and nonhygroscopic. Care must be taken in work with the perchlorates because in dry form they can undergo a violent explosion if touched, even with a soft Teflon spatula (94UP1). 

An interesting synthetic approach was reported in 1994. In this reaction, A-fluoropyridinium salts (137) were treated with nitriles in the presence of cyanate ion, affording pyridotriazines (138) together with the amides (139) (Equation (23)) <94TL207>. A carbene, derived from (137), was proposed as intermediate. 

An early report described the preparation in low yield of two isomeric meso-monofluorinated derivatives of porphyrins using the Schiemann reaction. Electrophilic fluorinating agents have been a more effective way to carry out direct fluorination of the porphyrin system. For example, reaction of octaethylporphyrin with A-fluoropyridinium salts such as A-fluoro-2,3,4,5,6-pentachloropyridinium triflate gave a mixture of mono, di, tri, and tetrafluoro derivatives 88 (Fig. 3.41). The perfluorinated derivative was formed in 20% yield. mew-Fluorination of octaethylporphyrin had minimal effects on oxidation potential and spectral properties. 

TABLE 6.4 Preparation of 2-Fluoropyridines 13 from A-Fluoropyridinium Salts 12 ... 

A/ -Fluoropyridinium salts and their analogs as fluorinating agents 96MI6. 

Some of the widely accepted fluorinating agents such as A -fluoropyridinium salts proved to be absolutely inert in these reactions (Scheme 38) (97JOSC301, 86BCJ3625, 95JOC6563). Therefore, electrochemical oxidation can be understood as an alternative to known fluorination procedures, and one can expect that this method will be further developed and applied to other organic substrates. 

The cine-substitution reactions in A -fluoropyridinium salts with various nucleophiles, including iV-nucleophiles, have been reviewed [121]. However, this approach did not find practical applications because of low yields of 2-substituted pyridines and the formation of complex mixtures of reaction products. 

A novel method of generating carbocation intermediates has been developed, which utilizes a cobalt complex, A-fluoropyridinium salt, and silane (Scheme 13). The chemistry has been used to prepare hydroxalkoxylation products (51) from olefins by Markovnikov addition. A mechanism has been proposed in which a cobalt hydride species adds to the olefin to provide intermediate (52). Initial formation of the carbon-centered radical (53) is followed by oxidation to the carbocation (54). Subsequent trapping of the carbocation leads to the hydroxalkoxylation product (51). 

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